The present invention relates to a method of operating an exhaust-gas-turbocharged, low-compression multi-cylinder Diesel-cycle internal combustion engine that has a first cylinder group which operates over the full load range, and a second cylinder group which is cut out during starting or at no load (idle) and in the part load range, but is cut in in the remaining load range. Cutting out and cutting in of the second cylinder group is effected by cutting off or cutting in the fuel supply, each cylinder group having associated with it an exhaust-gas-operated turbocharger with an associated charge air pipe, the charge air pipes communicating with each other through a so-called pressure equalizing system.
A disadvantage of supercharging consists in the fact that the ignition pressure rises, and hence the mechanical and thermal stresses in the piston/connection rod/crankshaft system increase. To prevent this effect, supercharging must be accompanied by a corresponding decrease in the compression ratio of the engine. The resultant decrease in the final compression temperature adversely affects the ignition conditions in the cylinder and influences the starting and no-load behavior, including operation in the low-load range. Excessive formation of white smoke is the consequence.
White smoke contains unburned hydrocarbons (i.e. fuel droplets) which are emitted with the exhaust gases from the cylinders. It is the result of incomplete combustion in the cylinders and occurs when the compression temperature in the cylinders of the engine is not sufficient to burn all of the injected fuel, such as is the case, for instance, when the engine is being started.
White smoke thus differs in its substance from black smoke, which contains carbon particles and which occurs under conditions where the compression temperature is sufficient to ensure combustion, but the supply of oxygen is not sufficient to oxidize all of the carbon in the fuel.
To improve this critical starting and part-load behavior, it is known to cut out a certain number of cylinders (i.e. there will be no injection into these cylinders). This method is generally known. It is applied to Otto-cycle engines to decrease the fuel consumption in the low-load range. The importance of this method for the supercharged Diesel engine is in the fact that cutting out of cylinders will place an additional load on the remaining firing cylinders. This means higher volumetric efficiency, increase in the amount of fuel injected, improved atomization or mixture formation, and a higher temperature level in the working cylinders. The result is a substantial reduction in hydrocarbon emission.
The (straight) cylinder cutout has been applied to Diesel engines--as far as is known--only in no-load operation and in the transitional phase to low mean effective pressures. To this end, cutout control systems have been developed both for engines with unit and individual injection pumps (see, for instance, DE-PS 28 21 161, key word: Split control rod).
Where further reduction in the compression ratios is desired, this problem is solved by combining cylinder cutout with post-charging of firing cylinders by pre-compressed air from the non-firing cylinders (see, for instance, German Pat. Nos. 33 22 168 and 33 23 337).
This method calls for a considerable control complexity, including valves, air reservoirs, as well as cutting-out and cutting-in devices for the turbochargers concerned.
The present invention therefore relies only on straight cylinder cutout, which itself is known. However, in the case of the type of internal combustion engine preferred to above with two separate turbochargers and two charge air pipes, which are interconnected by a pressure equalizing circuit, this has a serious disadvantage in poor acceleration and increased black smoke in the cutout mode because the charge air pressure built up by the relevant turbochargers via the firing cylinders expands instantly via the pressure equalization into the non-firing cylinder bank. As a result, the charge air pressure in the firing cylinders decreases and, consequently, is not available during combustion. As a result, a smoke stop provided on the injection pump (LDA=charge pressure-dependent full-load stop, which limits the control rod path on the injection pump and, consequently, the injection rate at low charge pressures) releases the amount of fuel needed for rapid acceleration only with such a time lag as is required for the necessary charge pressure to build up again or, failing an LDA fuel stop, would lead to substantial amounts of black smoke.
The object of the present invention is therefore to offset deficient supercharging by the turbocharger in an internal combustion engine of the aforementioned general type during starting and at part load in a simple manner, and to achieve a sufficiently high temperature in the firing engine cylinders for ignition to occur despite the low compression ratio during starting, and/or better combustion of the fuel with reduced emission of noxious matter in the exhaust gas in the partial load mode.